This invention relates to a method and apparatuses for logging hydrocarbon reservoirs. More particularly the invention concerns the logging of gas/oil and water/oil contacts in geological formations.
It is highly desirable to be able reliably to distinguish between oil and water in a geological formation penetrated by an oil well. The ability to do so enables one to determine whether a porous formation contains oil or water and the position of the oil/water interface during well production and whether the water driving fluid has broken through to the production well in a water flood secondary production operation. In open-hole, that is a well without a steel liner or casing, it is conventional to distinguish between oil and water by means of a resistivity tool which reads a low resistivity when the formation is saturated with saline water, a good conductor; and a high resistivity when the formation is saturated with oil, an insulator.
It is also conventional to distinguish between oil and water in a cased well by taking advantage of the differences in the macroscopic neutron absorption cross section (Sigma) of oil and the normally saline formation water. Since the saline formation water contains chlorine which has a rather high neutron capture cross section and since oil does not, neutron tools have been developed which essentially measure the macroscopic neutron capture cross section.
For example, U.S. Pat. Nos. 3,566,116 (reissued Jul. 8, 1975 as U.S. Pat. No. 28,477); 3,691,378; and 4,055,763 illustrate variations of one such technique for determining Sigma in which a pulsed neutron source is utilized to irradiate the formation with a repetitive burst of fast neutrons in order to permit a time evaluation of the neutron population in the resultant neutron cloud. Typically, this evaluation is accomplished by detecting capture gamma rays which result when thermalized neutrons of the cloud are captured or absorbed by a nucleus of a constituent element in the formation. In such a time evaluation, advantage is taken of the fact that the neutron cloud density decays exponentially, with the characteristic decay time being a function of the macroscopic neutron absorption cross section of the formation. The macroscopic neutron absorption cross section is the sum of the neutron absorption of the elemental constituents of the formation and of its contained fluids.
While these neutron tools and techniques are quite effective in distinguishing between oil and water under normal circumstances, a number of limitations have been encountered. Once such limitation is the situation in which the non-oil fluid in the formation is fresh water rather than saline (or, more generally, chloride-containing) water. In this circumstance it is not possible using the above described pulsed neutron technique to distinguish between oil and water since the difference in neutron capture cross section between the two formation fluids (oil and fresh water) is not large enough to permit their differentiation.
An additional limitation with the pulsed neutron technique is encountered in wells that have fresh water in the well borehole, even though saline water is present in the formation. In such a circumstance, some neutrons from the neutron burst are thermalized and linger in the fresh water of the borehole, giving rise to an interfering xe2x80x9cdiffusionxe2x80x9d background. This effect of course does not occur in those boreholes having saline water since the chlorine is a strong neutron absorber which rapidly absorbs the diffusing neutrons. The xe2x80x9cdiffusionxe2x80x9d background is a particularly awkward problem in the pulsed neutron technique since the determination of the characteristic decay time following the neutron burst relies on the detection of neutron fluxes whose intensities decrease with time to relatively small values. As a result, the xe2x80x9cdiffusionxe2x80x9d background becomes large relative to the neutron flux of interest so as to obscure the information bearing signal.
In view of the difficulties and limitations inherent in the pulsed neutron technique, other measurements and techniques that might be suitable for distinguishing between oil and water were sought. Another conventional neutron instrument used in logging oil wells is commonly referred to as the neutron-neutron tool since it contains a continuous neutron source for irradiating the formation and neutron detectors for detecting the spatial distribution of neutrons established by the source. It is conventional to utilize this tool to measure porosity of the formation under investigation. U.S. Pat. No. 3,483,376, describes in detail an illustrative embodiment of such a neutron-neutron tool.
Interestingly, in the past, very little has been understood about which parameters of a medium influence porosity response in an investigating instrument. This is indeed the case for neutron-neutron or neutron-gamma porosity tools. Such neutron tools utilize a source for emitting neutrons into the adjacent formations and subsequently or simultaneously detect the spatial distribution of the resultant neutron cloud through either the direct detection of neutrons or through the detection of gamma rays which are created when a neutron is absorbed in the nucleus of an atom of the formation.
Following emission from the source, the neutrons travel through the formation and lose energy by collision with the nuclei of the atoms of the formation. When the energy level of the neutrons is reduced or moderated sufficiently, they may be detected and counted by the investigating instrument. Generally, it is assumed that primarily the hydrogen index (i.e., the number of hydrogen atoms per unit volume of the formation fluid) is responsible for the spatial distribution of the cloud of neutrons. Since hydrogen is the only element whose nuclear mass resembles that of the neutron, hydrogen is the most effective element in reducing the energy level of the neutrons to a level at which they are eventually detected. In general, the formation pore spaces are filled either with water or with liquid hydrocarbons which both contain hydrogen. Thus, this type of neutron log is essentially a record of the hydrogen atom density of the rocks surrounding the borehole. Previously, the neutron log has been considered, therefore, to be a measure of the formation porosity. It is well recognized that gas, on the other hand, will alter this porosity determination since the gas is much less dense than its oil liquid counterpart.
U.S. Pat. No. 4,095,102 compares a value of porosity derived from an epithermal neutron-(gamma) tool with a value of porosity derived directly from a measurement of the thermal neutron absorption characteristic of the formation and the value of the water component of the formation. Where a difference is noted, hydrocarbon may be expected. In a manner similar to those techniques described earlier that utilize pulses of neutrons to determine a characteristic decay time dependent on macroscopic neutron capture cross section and hence a porosity, the disclosed technique requires saline water in the formation.
U.S. Pat. No. 4,416,151 also compares values of porosity derived from two different methods of porosity measurement, and associates differences with the presence of hydrocarbon.
During the life of an oil well the amount of water in the well itself tends to increase, and the above inventions can be sensitive to this, an effect which may mask the reliable detection of water/oil contacts in the reservoir, compromising their value.
According to a first aspect of the invention there is provided a method, of logging a borehole, for use in a cased oil well for detecting the gas/oil or water/oil contact in a hydrocarbon reservoir, using a neutron capture technique to detect one or more properties of fluid by comparing the count rates from two or more radiation detectors spaced at respective first and second distances from an isotopic source of continuous neutron radiation, the method including the steps of, at one or more locations along a said borehole, independently determining the properties of the borehole fluid and the formation fluid to define contact detection criteria that are substantially unaffected by the borehole fluid and formation fluid chloride contents.
Advantages of this method are:
1. It avoids the problem arising from the xe2x80x9cdiffusionxe2x80x9d background radiation caused by the borehole fluid;
2. It is highly effective regardless of the salinity of the formation and borehole fluids;
3. It may be practised using a compact tool that if desired may be self-powered and need not be connected to surface location via a logging cable.
Preferably the detected fluid property includes the density of the fluid in the reservoir.
It is believed that the method of the invention may also be used to detect other properties of the fluids in the formation.
Preferred embodiments of the method of the invention include the steps of comparing the counts generated by the said detectors against a characteristic plot of count rates corresponding to known borehole and formation fluids, whereby to ascertain the characteristics of the formation fluid. The characteristic plot preferably includes a plurality of substantially constant gradient curves representing the relative proportions of water and oil or gas and oil in respectively the formation and borehole fluids; and the step of comparing the generated count rates against the plot includes the step of mapping a generated count point onto one of the constant gradient curves at a point on the characteristic plot that corresponds to a characteristic of the formation fluid.
The use of such a characteristic plot, and such a method of analysing the outputs of the radiation detectors, provide highly sensitive logging of the gas/oil or water/oil contact in a formation.
More specifically the constant gradient curve onto which the generated count point is mapped preferably represents the relative proportions of water and oil (or gas and oil) in the formation fluid. Such a constant gradient curve may be stored eg. as a look-up table in the memory of a digital computer programmed to execute the method of the invention.
More specifically still, the step of mapping the generated count plot includes migrating a plotted generated count value along a constant gradient interpolation line that is interpolated between and intersects the intersection of two divergent constant gradient curves of the characteristic plot.
This technique advantageously lends itself to execution by an iterative method, or by an analytical method, either of which may be carried out by a suitably programmed digital computer.
In particularly preferred embodiments of the method of the invention, the characteristic plot is a two dimensional plot of count rates generated by the respective detectors.
Conveniently the method includes the step of generating a signal indicative of the relevant proportions of water and oil or gas and oil in the formation fluid. Such a signal may be used e.g. to store data as a log of the formation; or to transmit data e.g. to a remote location for analysis.
Consequently one embodiment of the method includes use of a borehole logging tool, and includes the step of transmitting the said signal from the tool at a downhole location to a surface location or another location remote from the logging tool.
In an alternative embodiment of the method the transmitting step includes storing data representative of the signal in a data storage device operatively connected to or forming part of the well logging tool; retrieving the data storage device to a location remote from the downhole; and downloading the data from the data storage device e.g. for subsequent analysis and/or interpretation.
As an alternative to the aforesaid steps, the transmitting step may include transmitting a signal in real time from a downhole location to a location remote from the logging tool such as a surface location. Transmission of the signal may conveniently be achieved by one or more of electrical conduction via one or more wires; or generation of pressure pulses in borehole fluid.
Conveniently the logging is carried out using a compact diameter (i.e. less than 2xc2xc inches (57 mm) diameter) battery/memory tool (i.e. a tool having an on-board memory and power supply, that does not require power and data transmission cables to enable logging to take place). In particularly preferred embodiments of the invention the logging tool may if desired be the same (or of the same design) as the logging tool used to generate an openhole log of the borehole before deployment of a casing.
According to another aspect of the invention there is provided apparatus for detecting the gas/oil or water/oil contact in a hydrocarbon reservoir penetrated by a cased oil well, the apparatus comprising:
(i) a logging tool having: a pressure case; a continuous source of fast neutrons; and two or more radiation detectors each spaced from the source by a different distance;
(ii) a computer for coordinating a method as defined herein; and
(iii) a transmitter for transmitting one or more signals characteristic of the gas/oil or water/oil contact in a formation.
This apparatus is advantageously suitable for carrying out the method of the invention defined herein.
In one embodiment of the invention the radiation detectors include two gamma detectors that are spaced from one another in the logging tool.
In another embodiment the radiation detectors include two neutron detectors that are spaced from another in the logging tool.
A further possibility is a logging tool that includes both neutron and gamma detectors.
The logging tool may be of the conventional type that includes a logging cable operatively connected thereto, for suspending, powering and permitting data transmission to and from the logging tool.
As an alternative the logging tool may include one or more power sources and data storage devices. The latter kind of logging tool may be configured as a so-called xe2x80x9cbattery memory toolxe2x80x9d, that does not necessarily require electrical connection to a surface location, being self-powered and capable of storing data until an opportunity for downloading the data arises following recovery of the logging tool to a surface (or other) location. Preferably such a tool is of so-called xe2x80x9ccompactxe2x80x9d diameter (i.e. less than 2xc2xc inches (57 mm) diameter).
In particularly preferred embodiments of the invention, the computer that co-ordinates execution of the method is within the pressure casing of the logging tool.
In summary a method is described which utilizes a logging tool that examines the count rates acquired in a novel way to determine a change in formation fluids without the effects being masked by changes in the borehole fluid. Over a period of time, as an oil well produces oil, the contact between water and oil changes, as do the fractions of water and oil in the well bore itself. If well logs acquired over this period are examined, these two changes are the only significant changes that can have occurred, but it is difficult to distinguish them from one another. The method and apparatus of the invention allow assessment of the change in formation fluids to be made. It has been shown that oil and low salinity water affect the measured neutron flux at the detectors differently, allowing the method to be used.
The invention also resides in data obtained by practicing the method of the invention defined herein.